Computers typically have a number of different types of memory devices for storing data. For example, a read only memory (ROM) is a semiconductor memory device in which data is permanently stored. The data cannot be overwritten or otherwise altered by the user. A ROM is also non-volatile which means that the data is not destroyed when power is lost. A ROM is "programmed" during its manufacture by making permanent electrical connections in selected memory cells. ROMs are useful wherever unalterable data or instructions are required. A disadvantage of ROMs is that their programming is determined during the design stage and can only be varied by redesigning the chip.
An alternative to a ROM is a programmable read only memory (PROM) which is programmable once after its manufacture. In one type of PROM, each memory cell is provided with an electrical connection in the form of a fusible link (fuse). There are a considerable number of known designs for fuses used in PROMs. Perhaps the most common is a metal link, such as tungsten, which is narrowed or necked down in one region. To blow the fuse, current is driven through the link. The current heats the link to its melting point and the link is broken. Usually, the link breaks in the necked down region because that is where the current density is highest.
A problem with existing fuse designs is referred to as "growback" or "metal migration". Growback is a phenomenon in which a fuse metal which has been blown grows back together to reconnect the fusible link. Although the physics of growback is not completely understood, if the link reconnects, data is lost--clearly an undesirable result.